This invention relates to the field of telecommunications and more specifically to a method and apparatus for determining crosstalk.
As demand for broadband services, such as high speed Internet access and on-demand video to the home increases, the need for technologies that can transmit large bandwidth of information also increases.
One technology being introduced to fill this need is digital subscriber lines (xDSL). The advantage of xDSL over other broadband technologies such as fiber optics to the home or cable modems is that it uses the existing twisted pair copper lines that already provide telephone services to homes to provide high bandwidth transmissions.
The term xDSL is a catchall term covering a number of similar technologies. These include High bit rate Digital Subscriber Line (HDSL) and Asymmetric Digital Subscriber Line (ADSL).
HDSL is designed to deliver T1 (1.544 Mbps) and E1 (2.048 Mbps) services over unconditioned copper lines by using transceivers on each end of two or three twisted pairs. Single pair HDSL is typically limited to 384 Kbps or 768 Kbps symmetrical speeds.
ADSL also uses twisted pairs of copper lines to provide a large downstream path and a smaller upstream path. This allows for services such as on demand video and high speed Internet access where the user needs to receive greater bandwidth than the user needs to send. In one embodiment, ADSL is capable of providing downstream rates in excess of 6 Mbps and simultaneous duplex transmissions of 640 Kbps. Several competing ADSL standards exist. These include Discrete Multitone (DMT) and carrierless Amplitude and Phase modulation (CAP). In one embodiment, DMT divides the 1 MHZ phone line spectrum into 256 4 KHz channels. Bit density can be varied to overcome noise and interference.
CAP uses a single carrier and utilizes amplitude modulation similar to that used for modems.
Both DMT and CAP operate by placing an ADSL modem on either end of a twisted-pair telephone line (one at a central office and the other at the customer""s home or office). Three channels are created: a POTS (plain old telephone service) channel, a medium speed duplex channel and a high speed downstream channel. In a typical implementation, POTS takes up the first 4 KHz of the phone line spectrum. The medium speed duplex channel and the high speed downstream channel occupy higher frequencies in the spectrum. Since the POTS channel is located in the first 4 KHz of the spectrum it can be split off from the data channels by filtering, thus providing an uninterrupted POTS connection.
The phone lines that ADSL is provided on are regular twisted pair lines. These are often provided as approximately 25 lines in the same bundle. When the lines are close together, crosstalk can occur. Crosstalk is caused by the signal in one line causing noise in another. The noisier the line, the lower the possible transmission rate across the line. To help fix this problem, margin, measured in terms of decibels, is used to pad the transmission rate. The margin is currently based on white noise or flat noise. For example, if a receiver requires a 25 dB signal to noise ratio (SNR) for acceptable transmission at a rate, that required SNR number is increased by 1 dB to give 1 dB of margin. However, if the noise induced on the line is not white noise (which it rarely is), the margin would be higher than necessary which, in effect, artificially reduces the bandwidth for transmission.
Accordingly, a need has arisen for a method and apparatus for determining and compensating for crosstalk. The present invention includes a method and apparatus for determining and compensating for crosstalk that addresses the shortcoming of prior systems and methods.
According to one embodiment of the invention, a system to analyze and compensate for noise on a transmission line is provided. The system comprises a transmission card which includes a transceiver, and a microprocessor attached to the transceiver. The system also comprises a transmission line coupled to the transceiver and at least one customer premise equipment coupled to the transceiver by the transmission line. The microprocessor is operable to monitor the transmission line to provide a margin between the noise on the line and a transmitted signal
The present invention provides various technical advantages. Noise can be detected on an ADSL line and transmission adjusted according to the noise measured and to the type of noise expected in the future. Other technical advantages may be readily apparent to one skilled in the art from the following figures, descriptions and claims.